Recently, much effort has been directed to developing advanced direct coal-fired gas turbine systems for electric utility applications. One approach is based on a multi-stage, slagging combustor with specified emissions control. Instead of requiring a low-ash, highly beneficiated coal, the slagging combustor concept allows the use of a raw or moderately cleaned, utility grade coal of higher ash content, thus maintaining a lower fuel cost. The economics of such a design are considered more favorable than conventional pulverized coal steam plants.
The success of direct coal-fired combustion systems relies heavily on the effectiveness of the slagging combustor itself. The slagging combustor is a modular unit with three principal stages. The first stage is the primary combustion zone or compartment. Coal and pre-heated compressed air, enter the primary zone coaxially through a plurality of injection nozzles which are equally spaced around the combustion chamber. This coaxial injection promotes intense air and coal mixing and rapid particle heat-up and devolatilization, which minimize carbon burn-out time.
The incoming coal-air jets converge at the combustor center line and form one vertical jet which impacts the combustor dome. This forms a toroidal vortex which provides the mechanisms for flame stabilization and centrifugal separation of larger ash and slag particles. The separated ash and slag form a stable flowing layer on the combustor walls. The vertical geometry of this stage allows gravity to assist in the removal of the molten slag. Fuel-rich conditions in the primary combustion zone inhibit NO.sub.x formation from fuel-bound nitrogen. They also provide the necessary reducing conditions for removal and capture of sulfur. The sulfur sorbent, limestone, dolomite, hematite or magnetite is counter-flow injected into the downstream end of the primary zone. The first stage is designed for a coal particle residence time of about 100 ms (for 75 micron sized particles).
The second stage of the direct coal-fired combustor is an efficient impact separator which is closely coupled to the first stage. The impact separator removes particulates carried over with the gas from the primary zone, whether they are sorbent or fine particles of ash.
To meet government regulations regarding particulate standards, a slagging cyclone separator has been suggested to be combined with the impact separator. See Diehl, et al. "Development of an Advanced Coal-Fired Gas Turbine Combustor" AVCO Research Laboratory, Heat. Engine Contractors' Conference, June 14, 1988; Loftus, et al. "The Use of 3-D Numerical Modelling in the Design of a Gas Turbine Coal Combustor", AVCO Research Laboratory, presented at the Winter Annual Meeting of the American Society of Mechanical Engineers (Dec., 1988), which are hereby incorporated by reference. This combination of slag and ash removal promotes cleaner inlet gases for the third stage, also referred to as the secondary combustion zone. However, significant heat loss occurs, since the gases are channeled over a relatively large expanse of cooler surfaces of an additional pressure vessel, containing the cyclone separator, located between the exit of the impact separator and the entrance of the secondary combustion zone. Although the additional pressure vessel is water-cooled and refractory-lined for receiving the high temperature gases leaving the impact separator, it causes a substantial additional heat loss from the cycle. Moreover, the combination of impact and cyclone separators necessitates the use of a larger pressure vessel environment, with attendant expense and maintenance.
For slagging to occur, temperatures throughout the combustion chamber, and particularly at the walls, must not fall below about 2,600.degree. F., 1427.degree. C., depending upon the type of coal. These temperatures are achieved by employing a proper air to fuel ratio, by providing adequate design for refractory walls, and sufficient residence time for the reacting coal to air mixture.
In the third stage of the slagging combustor, complete combustion is accomplished and the combustion gases are tempered to meet turbine inlet requirements. This stage preferably consists of a single module with a pressure shell as is found in the first two stages. To prevent the possibility of refractory particles in the exhaust system passing to the turbine, this section has no refractory lining. Combustion and dilution air are introduced through two rows of injection tubes with approximately sixteen tubes per row circumferentially spaced.
While for the most part, the early designs of direct coal-fired gas turbine systems have showed much promise, there is a current need for a more effective method for removing slag particles and ash prior to forwarding gases into the secondary combustion zone. Since gas turbine systems often operate at high pressures customary for the industry, i.e., 10, 12, or 14 atmospheres, any deslagging procedure should minimize system high pressure working fluid losses. Such a system would also be required to move a high volume of slagging products out of the gas turbine within a relatively short time, with a minimum of heat loss for maintaining efficiency.